1. Field of Invention
At least one embodiment of the invention relates to an uninterruptible power supply (“UPS”) and, in particular, to a UPS for operation when connected to an AC source that does not provide a neutral conductor.
2. Discussion of Related Art
Uninterruptible power supplies are often employed to supply more reliable power to one or more electrical loads, for example, critical loads. Typically, in an online UPS, the UPS converts an AC input to DC and supplies the DC to circuitry in the UPS that converts the DC to an AC output connected to the loads. In addition, a UPS typically includes batteries that supply power during periods when the AC input is unavailable. Polyphase UPSs employing power factor control are well-known today. Such UPSs typically are connected to a polyphase AC input that includes a neutral. Generally, the UPS includes a continuous neutral connection from the UPS input to the UPS output. In many of these known approaches, the batteries that are employed with the UPS are connected to the neutral.
A high level schematic of a power converter used to convert AC to DC, for example, in a UPS, is shown in FIG. 1. In one embodiment, an AC source 100 is connected to a rectifier 102 included in a UPS. The UPS typically also includes input capacitors 104 that may be employed as filter capacitors to eliminate electrical noise that would otherwise be transmitted from the UPS to the AC source 100. The rectifier 102 is connected to a plurality of boost converters 115A, 115B, 117A, 117B, 119A, 119B included in circuitry 106 that convert the rectified AC to DC which is supplied to each of a positive DC bus 108 and a negative DC bus 110. As is shown in FIG. 1, the UPS also includes a neutral 112. Each of the positive DC bus 108, the negative DC bus 110, and the neutral 112 are supplied to further UPS circuitry (e.g., to an inverter) that converts the DC to an AC output voltage at the output of the UPS. For purposes of clarity, the circuitry used to convert the DC to AC, which is well known to those of skill in the art, is not shown in FIG. 1.
The input capacitors 104 are each connected from a line (e.g., one of lines L1, L2, L3) to a common point 113 that is connected to the neutral 112 and a neutral 114 of the AC source 100. Thus, the neutral 112 of the UPS is connected to the neutral 114 supplied from the AC source 100.
The UPS includes batteries, for example, a first battery 101 that is configured with a negative battery potential connected to the neutral 112 and a positive battery potential connected to the input of boost converter 115A via a switch 103 (e.g., a silicon controlled rectifier). A second battery 105 is configured with a positive battery potential connected to the neutral 112 and a negative battery potential connected to the boost converter 115B via a switch 107.
Operation of the circuitry is well-known to those skilled in the art and is described in greater detail, for example, in International Application No. PCT/DK02/00041, filed on Jan. 22, 2002, by American Power Conversion Denmark APS, the disclosure of which is incorporated herein by reference.
Briefly, each phase of the AC source 100 (e.g., lines L1, L2, L3) is rectified to provide, for each phase, a positive half-cycle of the AC input and a negative half-cycle of the AC input. Two boost converter circuits (e.g., circuits 115A, 115B) are employed for each phase to operate during the positive half-cycle of the AC input and the negative half-cycle of the AC input, respectively. Each boost circuit associated with lines L2, L3 (e.g., boost circuits 117A, 117B, 119A, 119B) is substantially identical and a description of the operation of only boost circuits is 115A and 115B provided here. Boost circuit 115A includes an inductor 116A, a switch 118A (e.g., a transistor) and a diode 120A. The inductor 116A is switchably connected to the neutral 112 by the switch 118A to store energy in the inductor during a first period of an operating cycle. In a second period of the operating cycle, the inductor 116A is disconnected from the neutral 112 when the switch 118A is turned off. When the inductor 116A is disconnected from the neutral 112, the energy stored in the inductor is provided to the positive DC bus 108 via a diode 120A. During the period when the inductor 116A is providing energy to the positive DC bus 108, a capacitor 122 is also charged.
During the negative half cycles of the line L1, boost circuit 115B which includes an inductor 116B, a switch 118B, and a diode 120B, operates in a fashion similar to that described for the circuit 115A to provide power to the negative DC bus 110. Each of the remaining boost circuits operate in a similar manner to supply power to the positive DC bus 108 and the negative DC bus 110 during the respective positive and negative half-cycles of each line, for example, where boost circuits 117A and 119A supply power to the positive DC bus 108, and boost circuits 117B and 119B supply power to the negative DC bus 110. Operation of the switches that provide the switching in the circuitry 106 is provided by control logic that, in general, switches the switches on and off in response to a comparison between a desired output waveform and the existing waveform. Typically, operation of the boost converters is controlled by pulse width modulation. Further, the circuitry 106 may include power factor control to maintain a unity power factor of the power drawn from the AC source 100.
When the AC source 100 is unavailable, DC power from batteries 101, 105 provides power to the input of circuits 115A, 115B, 117A, 117B, 119A, 119B. Further, power from the batteries 101, 105 can be provided to circuits 115A, 115B when the AC source 100 is available to supplement the AC source, for example, during periods of heavy electrical loading.
Often, the load on the positive DC bus 108 and the negative DC bus 110 is balanced. There are circumstances, however, during which the two buses 108, 110 are unevenly loaded. For example, some UPSs employ separate battery chargers where a first battery charger charges the batteries that supply power to the positive DC bus 108 and a second battery charger charges the batteries that supply power to the negative DC bus 110. The separate battery chargers may draw different amounts of power, for example, where one charger is connected to a set of batteries that are discharged while the other charger is connected to a set of batteries that are partially or fully charged. The result of the unbalanced loading of the two buses 108, 110 is that some amount of DC current will flow in the neutral 112. As shown in FIG. 1, the DC current will return to the UPS input via neutral 112 and from there return to the AC source 100 via neutral 114.
In theory, the current drawn by each phase of the circuitry 106 should also be balanced because the circuitry generally operates as three current sources which draw currents having the same amplitude as one another at a 120° phase displacement relative to each other. In reality, however, component tolerances and other minor variations in hardware result in at least small unintended differences in either or both of the amplitude and the phase displacement of the current drawn in each of the boost circuits (i.e., 115A, 115B, 117A, 117B, and 119A, 119B) of the control circuit 106 when compared to the other two circuits. The unbalanced current draw results in a current in the neutral because the current drawn by circuitry 106 is not balanced from phase to phase. Here too, the neutral current will flow from the UPS neutral 112 to the AC source 100 via neutral 114.
Information concerning electrical system neutrals is presented here as background concerning some of the terminology used herein. Referring to FIG. 2, the system connections for a polyphase AC source supplied from a wye connected system and an AC source supplied from a delta configured system are shown in FIGS. 2A and 2B, respectively. As shown in FIG. 2A, a wye connected system includes a conductor for each phase L1, L2, L3 and a neutral point NP. The neutral conductor represented by the dashed line may be connected to the neutral point NP and be made available for connection to a load along with line conductors L1, L2, L3. The circumstances described herein where the AC source 100 does not include a neutral refer to the fact that a neutral conductor is not provided along with the line conductors L1, L2, L3.
Similarly, the delta system shown in FIG. 2B also includes a neutral point Np, however, the neutral point of the delta system is not physically embodied, and as a result, a neutral conductor cannot be connected to a delta-configured AC source in the manner shown for the wye-configured source. Because a neutral conductor is not obtained from a delta configured system, a neutral cannot be supplied to electrical equipment (including UPSs) along with line conductors L1, L2, and L3 where a delta system is used.
In a balanced polyphase AC system, the neutral point as shown with reference to the wye configured system in FIG. 2A is a point from which a voltage measured from each line conductor L1, L2, L3 has an equal magnitude relative to each of the other voltages measured from the remaining line conductors to neutral. That is, the same voltage exists at each of the three line terminals L1, L2, L3 with reference to the neutral point Np. In the delta connected system, the magnitude of voltages measured from the line conductors L1, L2, L3 to the delta neutral point Np also equal one another. Thus, although a physical location is not available from which a neutral conductor can be provided in a delta system, a neutral point does exist. (Referring to FIG. 1, the AC source is configured in a wye configuration with a neutral point Np.)
Further, although it may be advantageous to employ a neutral for improved safety (among others reasons) there are circumstances where the AC source 100 (whether a wye-configured source or a delta-configured source) does not include the neutral 114. Two of the more common examples are 480 volt delta connected AC sources employed in the U.S. and 3-wire 200 volt AC systems found in Japan. Because UPS systems are employed with AC sources 100 that do not include a neutral (e.g., 3-wire systems), it is desirable to connect a UPS with a neutral to an AC source that does not provide a neutral. However, typical control systems for UPSs do not provide satisfactory operation in such an installation.
For example, typically, a control system employed with the electronics of the boost circuits in the UPS includes a reference waveform generator. The control system includes a positive regulator for control of the positive half cycle of each of the three lines and a negative regulator for control of the negative half cycle of each of the three lines. An error signal based on the gain of the respective bus is received by the regulators as an input. That is, a difference between a positive gain and a reference is supplied to the regulator for the positive bus and a difference between a negative gain and a reference is supplied to the regulator for the negative bus. The regulator outputs are supplied as inputs to the reference waveform generator and outputs of the reference waveform generator are employed to control the switching of the boost circuits. During UPS operation it is desirable to maintain the positive DC bus and the negative DC bus substantially in balance. That is, the control system acts to maintain equal magnitude of the DC potential on the positive DC bus and the DC potential on the negative DC bus. As one example, the control system responds to a condition where the magnitude of the positive DC bus is less than the magnitude of the negative DC bus by increasing the amplitude of the positive half cycles of current supplied to the positive DC bus relative to the magnitude of the amplitude of the current supplied to the negative DC bus.
As a result, the output signals of the regulators determine the amplitude of the reference waveform that are supplied to controllers (e.g., PWM current controllers) used to control the current drawn from each line L1, L2, L3 by the circuitry 106.
Such an approach typically cannot maintain a balance between the voltage of the positive DC bus 108 and the voltage of the negative DC bus 110 where the AC source does not provide a neutral conductor for connection to the UPS neutral.
A separate transformer (e.g., a neutral transformer) is often used to derive a neutral at the UPS input where a UPS including a neutral is connected to an AC source that does not provide a neutral. Of course, this solution is expensive because an additional transformer is required. In addition, a neutral transformer requires more space to install and decreases overall system efficiency because of transformer losses. Some of these transformer losses generate additional heat that a cooling system must then remove to maintain a desired ambient temperature for UPS operation.